1. Field of the Invention
This invention relates generally to adjustment knobs and is concerned more particularly with a knob assembly having means for controllably locking a rotatable shaft in a selected angular position.
2. Discussion of the Prior Art
Control devices, such as valves, timers, rheostats, potentiometers, and the like, generally, are provided with a protruding shaft which may be rotated to adjust the device to a desired setting. A coaxially disposed knob may be affixed to a distal end portion of the shaft such that rotation of the knob produces a corresponding rotation of the shaft. Also, for critically adjusted devices, for example, the knob assembly may include suitable locking means for maintaining the rotatable shaft in a selected angular position until a new adjustment is required.
Thus, U.S. Pat. No. 2,787,353 granted to L. Spraragen discloses a push-to-turn knob assembly having a resiliently biased locking means engaging an encircled cam cylinder to prevent rotation of a bindingly attached shaft. The cam cylinder is encircled by a coextensive release cylinder which is actuated by an enclosing knob shell being pressed axially and rotated. A resulting initial rotation of the release cylinder relative to the cam cylinder disengages the locking means and permits subsequent rotation of the shaft to a newly selected angular position. When the knob shell is released, the locking means is resiliently urged into engagement with the cam cylinder to maintain the attached shaft in the desired angular position.
However, it may be found that due to a build-up of diametric tolerances, the release cylinder may be frictionally engaged by the enclosing knob shell, without any axial pressure being applied to the knob shell. As a result, inadvertent rotation of the knob may disengage the locking means and cause an unintended rotation of the shaft from a critical setting. Also, it may be found that a build-up of diametric tolerances produces sufficient frictional engagement between the release cylinder and the cam cylinder to prevent the locking means from being resiliently urged into engagement with the cam cylinder. Consequently, the cam cylinder and attached shaft will not be maintained in a critically adjusted angular position; and the purpose of the knob assembly will be frustrated.
Therefore, it is advantageous and desirable to provide a knob assembly having shaft locking means and releasing means which does not require a rotatable release cylinder having tight diametric tolerances.